Isolated Disk Galaxies Research Articles

Evolution and Destruction of Bars

We summarize here some ongoing work by our group on the formation and evolution of barred galaxies. We first discuss the evolution of bars in isolated disc galaxies. Then we address the problem of bar destruction during interactions and mergers. Finally we describe work on generation and regeneration of bars in interactions.

Forming stars on a viscous time-scale: the key to exponential stellar profiles in disc galaxies?

We argue for implementing star formation on a viscous time-scale in hydrodynamical simulations of disc galaxy formation and evolution. Modelling two-dimensional isolated disc galaxies with the Bhatnagar–Gross–Krook (BGK) hydrocode, we verify the analytic claim of various authors that if the characteristic time-scale for star formation is equal to the viscous time-scale in discs, the resulting stellar profile is exponential on several scalelengths whatever the initial gas and dark matter profile. This casts new light on both numerical and semi-analytical disc formation simulations that either (a) commence star formation in an already exponential gaseous disc, (b) begin a disc simulation with conditions known to lead to an exponential, i.e. the collapse of a spherically symmetric nearly uniform sphere of gas in solid-body rotation under the assumption of specific angular momentum conservation, or (c) in simulations performed in a hierarchical context, tune their feedback processes to delay disc formation until the dark matter haloes are slowly evolving and without much substructure so that the gas has the chance to collapse under conditions known to give exponentials. In such models, star formation follows a Schmidt-like law, which for lack of a suitable time-scale, resorts to an efficiency parameter. With star formation prescribed on a viscous time-scale, however, we find gas and star fractions after ∼12 Gyr that are consistent with observations without having to invoke a ‘fudge factor’ for star formation. Our results strongly suggest that despite our gap in understanding the exact link between star formation and viscosity, the viscous time-scale is indeed the natural time-scale for star formation.

Modelling star formation and feedback in simulations of interacting galaxies

ABSTRA C T We discuss a heuristic model to implement star formation and feedback in hydrodynamical simulations of galaxy formation and evolution. In this model, gas is allowed to cool radiatively and to form stars at a rate given by a simple Schmidt-type law. We assume that supernova feedback results in turbulent motions of gas below resolved scales, a process that can pressurize the diffuse gaseous medium effectively, even if it lacks substantial thermal support. Ignoring the complicated detailed physics of the feedback processes, we try to describe their net effect on the interstellar medium with a fiducial second reservoir of internal energy, which accounts for the kinetic energy content of the gas on unresolved scales. Applying the model to three-dimensional, fully self-consistent models of isolated disc galaxies, we show that the resulting feedback loop can be modelled with smoothed particle hydrodynamics such that converged results can be reached with moderate numerical resolution. With an appropriate choice of the free parameters, Kennicutt’s phenomenological star formation law can be reproduced over many orders of magnitude in gas surface density. We also apply the model to mergers of equal-mass disc galaxies, typically resulting in strong nuclear starbursts. Confirming previous findings, the presence of a bulge can delay the onset of the starburst from the first encounter of the galaxies until their final coalescence. The final density profiles of the merger remnants are consistent with de Vaucouleurs profiles, except for the innermost region, where the newly created stars give rise to a luminous core with stellar densities that may be in excess of those observed in the cores of most elliptical galaxies. By comparing the isophotal shapes of collisionless and dissipative merger simulations we show that dissipation leads to isophotes that are more discy than those of corresponding collisionless simulations.

Anisotropies in the Distribution of Satellite Galaxies

We find that satellites of isolated disk galaxies at projected radii between 300 and 500 kpc are distributed asymmetrically about the parent galaxy and aligned preferentially with the disk minor axis. The dynamical timescale at these radii is sufficiently long that the shape of this distribution must reflect the formation history of the outer halo rather than its internal evolution. We also find that the orbital angular momenta of satellites at projected major axis distances of 200 kpc tend to align with that of the central disk. These results demonstrate that satellites are dynamically related to their primary galaxy. Satellites drawn from current simulations of hierarchical galaxy formation exhibit neither the systematic alignment nor the net rotation with the central disk that we find in the data.

Cloud Collisions and Bulge Formation in N-Body Simulations of Disk Galaxies

view Abstract Citations (3) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cloud Collisions and Bulge Formation in N-Body Simulations of Disk Galaxies Klaric, Mario Abstract Simulations of zero-thickness self-gravitating disks composed of gas clouds and stars presented here show that cloud-cloud collisions play a key role in throwing clouds out of the disk plane. The clouds then scatter stars, which under the influence of the halo/disk potential end up forming a bulge. This process can account for the presence of bulges in relatively isolated disk galaxies, which could have also acquired bulges via satellite capture at an earlier epoch. But, since satellites do not seem so abundant, some kind of internal process must produce, or help produce, bulges. Based on these results, cloud-star scattering seems a very viable candidate. Publication: The Astronomical Journal Pub Date: June 1995 DOI: 10.1086/117468 Bibcode: 1995AJ....109.2522K Keywords: GALAXIES: INTERACTIONS; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: STRUCTURE full text sources ADS |

On the influence of bars and star formation on galactic abundance gradients

view Abstract Citations (188) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Influence of Bars and Star Formation on Galactic Abundance Gradients Friedli, D. ; Benz, W. ; Kennicutt, R. Abstract By means of three-dimensional numerical simulations, we have investigated the influence of gas flows driven by stellar bars and star formation on the abundance gradients in isolated disk galaxies. For axisymmetric disks, the abundance gradients evolve only slightly. The local mechanical energy injection associated with star formation is unable to flatten the gradient but produces a local abundance scatter of up to 0.2-0.3 dex in the gas. The stellar abundance gradient is flattened by mixing induced by velocity dispersion. The presence of a bar significantly alters the abundance distribution in both the stellar and the gaseous components. Over most of the galaxy, the slopes of the abundance gradients are reduced in a few dynamical timescales by more than 50%. At a fixed radius, scatter around the mean abundance is found to have two origins, a large-scale variation between arm and interarm regions of about 0.8 dex and an intrinsic, small-scale scatter of 0.4-0.5 dex. On the other hand, central starbursts resulting from the increased fueling of the nucleus by the bar strongly steepen the abundance gradients in these regions. Based on these results, we argue that some normal early-type galaxies with flat abundance distributions should either harbor a yet undetected bar or have had one in the past. The results also suggest that these galaxies may contain unusually metal- rich nuclei. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/187449 Bibcode: 1994ApJ...430L.105F Keywords: Abundance; Barred Galaxies; Galactic Evolution; Gradients; Three Dimensional Models; Gas Density; Gas Flow; Mass Distribution; Metallicity; Star Formation; Astrophysics; GALAXIES: ABUNDANCES; GALAXIES: EVOLUTION; GALAXIES: KINEMATICS AND DYNAMICS; ISM: KINEMATICS AND DYNAMICS; STARS: FORMATION full text sources ADS |

THE GLOBULAR CLUSTER SYSTEM OF THE EDGE-ON SB GALAXY NGC 5170

ABSTRACT A CCD study of the edge-on spiral galaxy, NGC 5170, was carried out with the CTIO 4-m telescope to search for its globular-cluster system (GCS). Three V frames near the galaxy and two removed from it (for background estimation) were analyzed. Following removal of galaxy light by median filtering and DAOPHOT analyses, a total of ~130 ± 20 presumed clusters were detected within 2 arc min (R=16 kpc at D=28 Mpc) of the galaxy center above a foreground of Galactic stars and a background of distant galaxies. There are two ways to interpret the luminosity function (LF) data. (a) Solving for all parameters of the Gaussian (assumed, from studies of other galaxies, to be the appropriate representation of the NGC 5170 GCLF) leads to σ=0.73 ± 0.21, a value rather less than commonly found. In this interpretation our data reach beyond the peak of the GCLF and the distance modulus inferred is (m-M) = 31.2 ± 0.3. However, this solution places strongest reliance on the faintest bins with the largest errors. (b) By fixing σ=1.3, the derived Gaussian falls shy of the GCLF peak and (m-M) = 32.3 ± 0.3. These determinations allow the first direct comparison between the GCLF method and the Fisher-Tully method, from which 32.5 ± 0.14 has been found. Adopting parameters arising from solution (b), and making all necessary corrections, we estimate a total cluster population of ~ 815 ± 320, a specific frequency, S = 1.3 ± 0.5, or (assuming a bulge-to-disk ratio of 0.5), Sspheroidal = 3.9 ± 1.5. The S values (which are robust with respect to the assumed distance) are consistent with what is seen for other isolated disk galaxies. A power-law fit to the radial density distribution of the cluster candidates yields an index of -1.69 ± 0.26, which is again consistent with parameters found by others for galaxies with total luminosity comparable to that of NGC 5170; to within the errors, the radial distribution of the halo light and clusters are indistinguishable.

IRAS observations of an optically selected sample of interacting galaxies

IRAS observations of a large, morphologically selected sample of strongly interacting disk-type galaxies have demonstrated that galaxy-galaxy collisions can lead to enhanced infrared emission, but not in all cases. Infrared luminosities of the interacting galaxies span a large range, but are about a factor of 2 higher, on average, than those of isolated disk galaxies. The data suggest the existence of a cutoff in blue luminosity, below which no galaxies show markedly enhanced infrared emission. Only the most strongly interacting systems in the sample show extreme values of infrared excess, suggesting that deep, interpenetrating collisions are necessary to drive infrared emission to extreme levels. Comparisons with optical indicators of star formation show that infrared excess and color temperatures correlate with the level of star-formation activity in the interacting galaxies. All interacting galaxies in our sample that exhibit an infrared excess and have higher than normal color temperatures also have optical indicators of high levels of star formation. It is not necessary to invoke processes other than star formation to account for the enhanced infrared luminosity in this sample of interacting galaxies.

Disk and spheroidal components of external galaxies: an overview

A brief review of current theoretical views on how disk and spheroidal galaxies form and evolve provides the background for a summary of recent optical observations of external galaxies. Primary emphasis is placed on a discussion of the large-scale distribution and chemical composition of the stellar and gaseous constituents of relatively isolated galaxies. New studies of halo and disk surface-brightness distributions in spiral and SO galaxies are summarized. The “missing mass” in galactic halos, the relationship between disk size and luminosity, the nonexponential character of disk light distributions and very low-surface-brightness disk systems are highlighted in this section. Next discussed are observations which may provide insight into the factors which regulate the star-forming history of galactic disks and the post-astration appearance of spiral galaxies. Finally, the observed properties of relatively isolated disk galaxies are compared with those located in dense groups. It appears from this comparison that environment plays a significant role in governing the evolutionary history of a galaxy.

Evolution of a stationary disk of stars

An improved potential solver for calculating the gravitational potential of isolated disk galaxies is presented. The potential solver is used to investigate the evolution of initially stationary axisymmetric disks of stars for various values of the initial velocity dispersion.

Simulation of the evolution of stellar systems

Computer models for isolated disk galaxies and for infi nite doubly periodic stellar systems are used to study the self-consistent motion of large numbers of interacting stars as they move in the plane of the galactic disk. The doubly periodic model is used to investigate the develop ment of the gravitational instability for a plane stellar system. The results obtained show that the initial con densations which form are about two Jeans lengths in diameter. An isolated, initially cold, balanced disk is found to be violently unstable. Adding a sciffcient amount of ve locity dispersion will stabilize all small-scale disturb ances. Most disks investigated are found to be unstable against slowly growing long wavelength modes, and after about two rotations the disks tend to assume a bar- shaped structure. It is also found that the final mass distribution for most disks can be closely approximated by an exponential variation irrespective of the initial mass distribution.